1. Field of the Invention
The present invention relates to the pulsed plasma generation of extreme ultraviolet (XUV) radiation and, more particularly, to the utilization of an ionized metal as the active medium, where the discharge therefrom is pulsed to create higher states of ionization of the metal. Selected ionization states of the metal may be utilized as a non-linear medium in which UV lasers may be frequency converted into coherent XUV radiation.
2. Description of the Prior Art
Since there are no primary laser sources available which can generate radiation in the XUV region, various methods are used to obtain these wavelengths. Optical frequency mixing has provided a source of narrowband coherent XUV radiation, as described in the article "Frequency Mixing in the Extreme Ultraviolet" by J. Reintjes appearing in Applied Optics, Vol. 19, No. 23, Dec. 1, 1980, at pp. 3389-3896. As discussed in the Reintjes article, coherent XUV radiation is obtained through harmonic generation and frequency mixing using rare gas halide lasers for generating the tunable XUV radiation. Rare-gas-halogen (RGH) lasers were also utilized as a source for obtaining XUV radiation in experiments by H. Egger et al reported in the article "Generation of High-Spectral-Brightness Tunable XUV Radiation at 83 nm" appearing in Optics Letters, Vol. 5, No. 7, July 1980 at pp. 282-284. Here, coherent XUV radiation was reported to have been produced by third-harmonic generation of a transform-limited-bandwidth KrF excimer laser in gaseous xenon. The observed XUV output, which was continuously tunable from 82.8 to 83.3 nm, had a peak power of 40 mW, a bandwidth less than 0.01 cm, and absolute frequency control to within 0.04 cm.
Another method used for the generation of incoherent XUV radiation is to employ a thin carbon foil as the target of UV radiation, where photoemission from the foil has been found to generate XUV radiation in the range 121.6 to 58.4 nm. A complete discussion of this method can be found in the article "Extreme Ultraviolet Induced Forward Photoemission From Thin Carbon Foils" by K. C. Hsieh et al. appearing in the Journal of Applied Physics, Vol. 51, No. 4, Apr. 1980 at pp. 2242-2246. However, this method is not preferred since provision must be made for translation or replacement of the foil since the laser shots degrade the target and each shot sees a different surface. This limits the repetition rate of the shots and adds complexity to the target chamber.
More recent work in this area has replaced the thin carbon foil with a liquid-mercury surface, where this surface does degrade even after tens of thousands of shots with a repetition rate of 10 Hz. One such exemplary method of generation XUV radiation in this manner is discussed in the article "Laser-Plasma-Induced Extreme-Ultraviolet Radiation from Liquid Mercury" by R. M. Jopson et al appearing in Optics Letters, Vol. 8, No. 5, May 1983 at pp. 265-267. As disclosed, incoherent XUV radiation from 100 to less than 30 nm is emitted from plasmas generated on a liquid-mercury surface. Since the target does not have to be translated, the target chamber is a simple device, where the mercury may be placed at the entrance of a vacuum monochromator. It is to be noted, however, that this emission is incoherent and can not be utilized as a laser source.
One of the persistent problems encountered in harmonic generation at deep XUV wavelengths is the background continuum absorption due to either the active medium or any background gases which may be present. If a long beam path is to be used, as in for example a 1 meter monochromator, this absorption becomes a critical problem. In the past, various methods such as differential pumping, have been used in an attempt to overcome this problem. In principle the problem can be avoided, or at least minimized, by physically confining the active medium to a small path length and utilizing completely evacuated monochromators in the XUV generation process.